When producing transparent materials such as, for example, polyethylene or polypropylene materials, it is extremely important that the material contains as few pollutants as possible. For this reason, it is previously known to carry out an optically based inspection of a film or a tape which is produced in the material in question, by means of which any possibly occurring pollutants can be detected.
A measurement system for such a type of inspection or measurement can, for example, comprise equipment with a CCD-scanner, which interacts with a suitable light source for inspection of the material in question.
For example, there can in the above-mentioned materials occur undesired pollutants in the form of non-homogeneities. These non-homogeneities can, for example, consist of oxidized material, foreign particles or so-called gels, which as such are transparent, but which have optical properties which deviate from the rest of the material. This can be detected by means of the above-mentioned equipment for optical inspection.
During optical inspection, defects can be detected by comparing the light transmission in each point with a predetermined threshold or the change compared to a previously stored reference value. If the measured light transmission exceeds or falls below this threshold, this is an indication of the occurrence of a defect in the current inspection point. This corresponds to a “triggering”, or “trig”.
During optical inspection according to prior art, the tape is inspected in successive sweeps or inspection lines, which normally run crosswise to the longitudinal direction of the tape, which is below referred to as the “y-direction”. The transversal direction with reference to the tape is below referred to as the “x-direction”. Each and every one of the sweeps will then generate information regarding the light transmission in each respective pixel. This information can then successively be transferred, and stored in a computer unit for later analysis.
Since the major part of a tape, which is intended for optical transmission will be free from defects, there is a need to limit the amount of measurement data generated by the CCD-scanner, and in this way sort out the large part of obtained data which thus is not interesting.
In a scanner of a known kind which is used for optical inspection, there is a data reduction by data which is generated at those sweeps which do not obtain a trig signal being discarded or written over. In other words the information regarding those sweeps which do not generate a signal which indicates that the current threshold value for light transmission has been exceeded/fallen below is ignored, which corresponds to a defect not having been found. This is done since there is no interest in a later analysis of measurement data which corresponds to a material without defects. If trigging does occur, the sweep in question is saved in a special buffer memory for later analysis in, for example, a PC. In this way, the amount of data which needs to be analysed by the PC is limited.
The above-mentioned threshold level must be set with a margin from the normal light transmission level which can be considered to correspond to a tape without defects. Due to normally occurring variations in the material and noise and disturbances in the measured signal from the CCD-scanner, the threshold value must thus be set at a sufficient distance from the level at which such disturbances occur. This means that those flanks in the y-direction which occur before and after those sweeps which generate a detection (trig signal) will never be included in the buffer memory, since they are often within the margin which is used between the normal measurement signal and the threshold value. This in turn leads to a risk of erroneous calculation of the size of the defects found, which of course is a problem in connection with determining the form and shape of the defect in the case of optical inspection.
When detecting small or transparent defects in moving lines, the sensitivity is also limited by signal variations in the x-direction which are not present in the y-direction. These signal variations can consist of illumination variations, dirt or dust on the optical components of the system, longitudinal lines, so-called day-lines, which occur when producing the tape/film which is to be examined, unevenness in the various pixels of the CCD-camera etc. The optical signal from these signal variations in the x-direction can even exceed the signal from those defects which are to be detected for further analysis.
If the variation of the signal level in the x-direction is used during detection, as is the case of the prior art, it is not possible to detect defects which are present at the start of the x sweep.